1. Field of the Invention
The present invention relates to a method of manufacturing an electrically conductive mayenite compound with a high electron density.
2. Description of the Related Art
A mayenite compound has a typical composition expressed as 12CaO. 7Al2O3, and has a characteristic crystalline structure including three-dimensionally connected cages, each having a diameter about 0.4 nm. A skeletal structure structuring the cages is positively charged, and forms twelve cages per unit lattice. Oxygen ions occupy 1/6 of the cages in order to satisfy an electrical neutrality condition of the crystal. However, the oxygen ions inside the cages have a chemical characteristic different from the oxygen ions composing the skeletal structure. Thus, the oxygen ions inside the cages are specifically referred to as “free oxygen ions”. The mayenite compound is also expressed as [Ca24Al28O64]4+·2O2− (Non-Patent Document 1).
When a part of or all of the free oxygen ions inside the cages of the mayenite compound are substituted for by electrons, the mayenite compound is provided with an electrical conductivity. This is because, the electrons included in the cages of the mayenite compound can freely move in the crystal without being tied by the respective cages (Patent Document 1). Such a mayenite compound with the electrical conductivity is referred to, in particular, as an “electrically conductive mayenite compound”.
Such an electrically conductive mayenite compound may be manufactured by, for example, a method of manufacturing by putting a mayenite compound powder in a carbon container with a cover and performing a heat treatment at 1300° C. under a nitrogen gas atmosphere (Patent Document 2). Hereinafter, this method is referred to as a “conventional method 1”.
Further, such an electrically conductive mayenite compound may be manufactured by a method of manufacturing by putting a mayenite compound in an alumina container with a cover with aluminum and performing a heat treatment at 1300° C. in vacuum (Patent Document 2). Hereinafter, this method is referred to as a “conventional method 2”.